Microstructure and Corrosion Resistance of ... - Corrosion Journal

CORROSION SCIENCE SECTION

Microstructure and Corrosion Resistance of Sputter-Deposited Titanium-Chromium Alloy Coatings D. Landolt, C. Robyr, and P. Mettraux*

ABSTRACT

INTRODUCTION

Titanium, chromium, and titanium-chromium alloy coatings were sputter-deposited to study their corrosion behaviors in relation to microstructure and composition. Silicon substrates were used to study the effect of alloying on intrinsic corrosion resistance of the coating materials, and brass substrates were used to study the effect of alloying on the penetrating porosity of the coatings. Corrosion behavior was characterized using linear sweep voltammetry. The crystal structure of the coatings was examined by x-ray diffraction (XRD) and the microstructure by scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) was used to estimate the real surface area of the coatings. Results showed alloying of titanium with chromium greatly influenced microstructure of the coatings. Alloying led to deposits of higher apparent density and, in some cases, to an x-ray amorphous structure. Alloy coatings showed significantly lower corrosion currents than the constituting metals. The effect was attributed to a smoother surface topography. When corrected for differences in real surface area, the intrinsic corrosion rate of the alloy coatings did not differ significantly from that of the constituting metals. Alloy coatings deposited on brass exhibited a lower porosity than titanium or chromium metal coatings produced under identical conditions.

Sputter-deposited alloy films have been reported to exhibit remarkable intrinsic corrosion resistance.1-23 Using sputter deposition, single-phase alloys can be produced under nonequilibrium conditions, and the concentration of corrosion-resistant alloying elements may exceed the thermodynamic solubility limit. Furthermore, the microstructure of sputterdeposited alloys can be varied over a wide range by changing deposition conditions.24-27 Columnar microstructures are observed most often, but finegrained26 or x-ray amorphous structures are obtained in some cases.14-18,28 As with most corrosion-resistant engineering alloys, sputter-deposited alloy films owe their corrosion resistance to the presence of a protective passive film. The corrosion behavior of magnetron-sputtered passive chromium alloys (e.g., chromium-zirconium, chromium-titanium, chromium-tantalum, and chromium-niobium alloys) were studied extensively by Hashimoto and coworkers.14-18 They found corrosion rates several orders of magnitude lower than that of their constituents. This effect was attributed to the amorphous structures of the alloys and the particular compositions of the passive films. When comparing the intrinsic corrosion resistance of coatings fabricated by physical vapor deposition (PVD), it is necessary to take into account microstructure-related differences in real surface area.29 PVD coatings of columnar microstructure usually exhibit a certain surface roughness, and therefore, their real surface area can differ signifi-

KEY WORDS: brass, chromium, electrochemical impedance spectroscopy, linear sweep voltammetry, porosity, silicon, sputter deposition, titanium, vapor deposition, x-ray diffraction Submitted for publication August 1997; in revised form, April 1998. * Laboratoire de Métallurgie Chimique, Département des Matériaux, Swiss Federal Institute of Technology, MX-C Ecublens, CH-1015, Lausanne, Switzerland.

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0010-9312/98/000167/$5.00+$0.50/0 © 1998, NACE International

CORROSION–OCTOBER 1998


cantly from the nominal (geometrical) surface area. Different methods for determination of electrode surface area have been discussed by Trasatti and Petrii.30 Recently, electrochemical impedance spectroscopy (EIS) also was used to determine the real surface area of titanium and chromium films fabricated by PVD.29 In EIS, the response of an electrode to a small sinusoidal perturbation of potential is recorded as a function of frequency. Regression of the measured impedance data using an appropriate model permits determination of the capacity of the electrolyte-electrode interface. For samples of the same kind, this capacity is proportional to the real surface area.29-30 Therefore, the impedance method is well suited for studying the effect of real surface area on intrinsic corrosion resistance of PVD films. Corrosion resistance of metallic objects protected by PVD coatings depends not only upon intrinsic corrosion resistance of the coating but also on porosity.31 Electrochemical direct current (DC) and alternating current (AC) methods for estimating the porosity of PVD coatings recently were evaluated and applied to characterization of titanium and titanium nitride (TiN) coatings on brass.32-33 Linear sweep voltammetry was particularly useful for estimating the porosity of coatings, which are electrochemically more noble than the substrate. In a given potential range, the measured anodic current depends directly upon the exposed surface area of the substrate. By comparing measurements on coated and uncoated samples under otherwise identical conditions, an estimation of penetrating porosity can be obtained. The objective of the present study was to investigate the effect of alloying on the intrinsic corrosion resistance and porosity of sputter-deposited titanium-chromium alloy coatings by comparison to the pure metals. For the study of the intrinsic corrosion resistance, the coatings were deposited on silicon, an inert substrate. For the porosity study, they were deposited on brass. Microstructure was studied by scanning electron microscopy (SEM) and by x-ray diffraction (XRD).

EXPERIMENTAL Coatings were deposited by DC magnetron sputtering in a laboratory apparatus (Edwards ESM 100†) equipped with a sample holder rotating at 18 rpm. Two planar magnetron sputtering sources were used, a titanium (99.99%) target and a chromium (99.95%) target. Rotation of the substrate holder ensured good chemical homogeneity of the deposits. Gas pressure in the chamber before admitting the argon was in the 10–6 mbar range, and purity of the argon working gas was 99.999%. For deposition of the titanium-chromium alloys, the power of the titanium cathode was †

Trade name.

CORROSION–Vol. 54, No. 10

kept constant at 400 W, and the chromium content was set by changing the power of the chromium cathode from 50 W to 400 W. No bias was applied, and the deposition pressure was kept constant to 1 x 10–2 mbar. PVD coatings were deposited on silicon wafers or on brass substrates. The coating thickness was ≈ 1 µ m. Before depositing the coating, the brass substrates were polished mechanically to a 1-µ m diamond spray finish. The substrates then were degreased, rinsed in water and isopropyl alcohol, and cleaned ultrasonically in trichlorotrifluorethane. After introduction into the deposition chamber, they were sputter cleaned by radiofrequency (RF) etching. Chemical composition of the coatings was characterized by x-ray fluorescence using a Kevex Omicron† microanalysis system equipped with a rhodium target. The crystallographic structure of the films was determined by low incident-angle XRD (Siemens 500†) and the microstructure by SEM. For measurement of electrochemical polarization curves, the film-covered samples were embedded in silicon resin and mounted on a rotating disk electrode (RDE) shaft. A nominal area of 0.2 cm2 was exposed to the electrolyte. All electrochemical experiments were performed at a rotation rate of 2,000 rpm at room temperature in deaerated 39 g/L sodium chloride (NaCl). Potentials were measured with respect to a saturated calomel electrode (SCE), but the reported values were converted to the hydrogen scale (SHE). EIS measurements were carried out under potentiostatic control at the open-circuit potential using a Zahner IM6† measuring system. Frequencies were varied from 0.1 Hz to 105 Hz, and the amplitude of the perturbation was kept constant to 10 mV. Electrical contact was made at the top of the films. Additional information about the experimental procedures has been given previously.29,34

RESULTS

Structure For the microstructure study, titanium and chromium metal coatings and titanium-chromium alloy coatings of different compositions were deposited on silicon substrates. The surface and cross-sectional micrographs shown in Figures 1 through 4 illustrate the microstructures of the different films. Cross sections were obtained by cleavage of the film-substrate system. Chromium metal coatings (Figure 1) exhibited a typical submicron-scale surface topography that reflected the growth mechanism, resulting in columnar grains. The relation between grain structure and surface topography for chromium films was evidenced further by the micrograph of Figure 5, which gives a three-dimensional view. The individual surface features observed corresponded to the growth columns. The microstructure of titanium films is

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(a)

(b)

FIGURE 1. SEM micrographs of chromium metal film on silicon: (a) surface morphology and (b) cross section.

(a)

(b)

FIGURE 2. SEM micrographs of titanium metal film on silicon: (a) surface morphology and (b) cross section.

illustrated in the micrographs of Figure 2. Again, a distinctly granular surface morphology was related to the columnar structure of the crystal grains, although the columns in this case were somewhat less defined than for chromium. The Ti-20% Cr coatings were distinctly smoother than the individual metal coatings (Figure 3). The cross section exhibited some features oriented perpendicular to the coating surface, but no individual columnar grains could be identified. Within resolution limits of the SEM, the surface of the Ti-50% Cr coating (Figure 4[a]) was perfectly smooth, with no growth-related features visible. The cleaved surface of the cross section (Figure 4[b]) exhibited distinct features parallel to the growth direction. However, these

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features were quite different from the growth columns of Figure 1(b) associated with crystal grains, and they were not reflected in the resulting surface morphology. Figure 6 shows XRD patterns of the four types of coatings. The titanium and the chromium films exhibited hexagonal close-packed (hcp) and face-centered cubic (fcc) structures, respectively. A broad peak, slightly shifted toward higher lattice parameters, was observed for the Ti-20% Cr coating. The Ti-50% Cr film exhibited no diffraction peaks, indicating an x-ray amorphous structure.

Intrinsic Corrosion Resistance Anodic polarization curves measured in 39 g/L NaCl for the different coatings deposited on silicon



(a)

(b)

FIGURE 3. SEM micrographs of Ti-20% Cr alloy film on silicon: (a) surface morphology and (b) cross section.

(a)

(b)

FIGURE 4. SEM micrographs of Ti-50% Cr alloy film on silicon: (a) surface morphology and (b) cross section.

are shown in Figure 7. Open-circuit potentials of pure titanium and of the alloys were near –450 mVSHE, while that of pure chromium was more anodic, at approximately –150 mVSHE. All coatings were spontaneously passive. Corrosion currents were determined by extrapolating the straight-line part of the anodic current observed in the semilogarithmic plot to the corrosion potential. In the case of titanium and of titanium-chromium alloys, the measured passive current increased steadily with potential. For chromium, a current plateau was observed. Values for the corrosion current are given in Table 1. The nominal surface area was 0.2 cm2 for all samples. Results of Table 1 showed the chromium and titanium metal films exhibited a corrosion current an order of mag-


nitude higher than that observed for the alloy coatings Ti-20% Cr and Ti-50% Cr. Similar differences in corrosion currents were observed previously for experiments conducted in 5 M hydrochloric acid (HCl).34

Real Surface Area The electrochemical impedance of mechanically polished bulk titanium and chromium electrodes (both polished to a grid 2,400 paper finish) was measured in 39 g/L NaCl. The Bode diagram of Figure 8 showed that variations of the phase and of the amplitude with frequency were similar for both materials. Impedance data were fitted using the electrical model shown in Figure 9, where CPE represents a constant phase element. This complex element is used fre-

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FIGURE 5. SEM micrograph of cleaved chromium film showing relation between surface topography and growth columns.

quently in EIS instead of a simple double-layer capacitance to account for the frequency dispersion observed.35 The total impedance of the system (Z) is given by Equation (1):

Z (ω) = Rs +

Rp 1 + RpjωC

α

(1)

where Rs is the ohmic resistance between the reference electrode capillary tip and the working electrode, and Rp is the polarization resistance (the value of which is governed by properties of the passive film). The interface capacity (C) in the present case includes contributions of the electrical double layer and the passive oxide film. The applied frequency is ␻, and ␣ characterizes the observed frequency dispersion, where 0 < ␣ < 1. For ␣ = 1, the CPE reduces to a simple capacitor. The value of ␣ may be affected by several factors, such as the roughness of the electrode, an inhomogeneous macroscopic current distribution related to cell geometry, the quality of the electrical contact to the electrode or an insufficient lateral conductivity of a thin film deposited on an insulating substrate.29 No systematic investigation was made of the origin of the frequency dispersion observed but care was taken to assure that the electrical contact to the oxide-covered thinfilm electrodes was good. All impedance data were fitted between 1 Hz and 104 Hz. At lower frequencies, a long acquisition time could lead to instability, especially in the case of the chromium coatings. At higher frequencies, electrical artifacts might interfere with the measurement. All samples studied showed essentially a capacitive behavior indicating Rp was very high. For fitting the experimental data, the value of

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FIGURE 6. XRD patterns of titanium, chromium, and titaniumchromium alloy films deposited on silicon.

Rp was fixed arbitrarily at 1012 Ω. As long as Rp was chosen sufficiently high, the fitted curves were not sensitive to the choice of the value for Rp. The impedance data were regressed using proportional weighting, which gave an equivalent importance to data at all frequencies. Results obtained from fitting the experimental data to the electrical model are given in Table 2. Values of Rs, ␣, and C obtained for the two metals were very close. Bode diagrams showing both experimental and calculated impedance data for PVD coatings are shown in Figure 10. All experiments were performed three times on each sample and on at least two samples of each sort. The standard deviation (␴c) of the capacitance then was calculated. The fitted curves represented by the solid lines in the figure were in good agreement with the experimental data for all coatings. Both pure titanium and chromium coatings exhibited a similar behavior, but a net shift in the curves was observed for the alloys: the threshold frequency at which the phase and amplitude increase shifted to higher values. Values of Rs, ␣, and C obtained from the fit are summarized in Table 3. As expected from theory, ␣ was closer to 1 for the smoother alloy coatings. Rs did not vary markedly from sample to sample. Values of C were 1 order of magnitude higher for the pure chromium and titanium films compared to the alloy films. Taking the relative value of the interface capacity as a measure of real surface area,29 the measured



TABLE 1 Corrosion Currents from Electrochemical Potential Sweep Experiments for Sputter-Deposited Metal and Alloy Coatings(A) Corrosion Current (mA) Titanium Chromium Ti-20% Cr Ti-50% Cr (A)

1 x 10–4 2.1 x 10–4 2.8 x 10–5 1.4 x 10–5

Electrode area = 0.2 cm2.

FIGURE 7. Polarization curves measured on sputtered titanium, chromium, and titanium-chromium alloy films deposited on silicon (electrolyte = 39 g/L NaCl, scan rate = 2 mV/s, electrode rotation rate = 2,000 rpm, and temperature = 25°C).

corrosion currents shown in Table 1 were corrected to account for differences in real surface area. Results are plotted in Figure 11. To establish this plot, it was assumed that, for the Ti-50% Cr film (which exhibited the lowest value of the interface capacity), the real surface area was the same as the geometrical surface area. This assumption was justified by the fact that the x-ray amorphous alloy film exhibited no measurable surface roughness (Figure 4[a]). Results of Figure 10 showed the described correction for real surface area had a drastic effect on interpretation of the corrosion behavior of the PVD films studied. Without correction for real surface, the two alloy films showed distinctly lower dissolution rates than the constituting metals, which was in agreement with findings of Akiyama, et al.2 When corrected for real surface area, however, the alloy coatings showed a corrosion rate similar to the pure metals.

FIGURE 8. Bode diagram for mechanically polished chromium and titanium bulk metals (electrolyte = 39 g/L NaCl, electrode rotation rate = 2,000 rpm, and temperature = 25°C).

Determination of Porosity Porosity of the coatings was studied on films deposited on brass substrates by measuring anodic polarization curves on the coated and uncoated substrate. For systems involving a passive coating on an electrochemically active substrate, the measured anodic current was proportional to the exposed substrate area through the pores and, therefore, could be taken as a measure of penetrating porosity.32-33 Figure 12 shows polarization curves measured for titanium, chromium, and titanium-chromium alloy coatings deposited on brass and for the uncoated brass substrate. The current peak at 0.2 V was typical for dissolution of brass in stagnant solution or through pores.32 It did not appear in the curve of the bare substrate in Figure 12 because these measurements were made using a RDE and forced convection tends to suppress diffusion-limited peaks in voltametric measurements. The data of Figure 12 showed


FIGURE 9. Electrical model for the interpretation of impedance data.

that the anodic currents measured for titanium and chromium coated samples were significantly higher than those measured in the presence of alloy coatings. This indicated the alloy coatings had a lower porosity, in agreement with what would be expected based upon the microstructures shown in Figures 1 through 4. Lowest anodic currents were observed for the Ti-20% Cr coating, rather than for the amorphous Ti-50% Cr alloy coating. This suggested that there was no simple correlation between the penetrating porosity and the degree of amorphicity in this case. However, at such low porosities, the reliability and reproducibility of electrochemical porosity

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TABLE 2 Model Parameters from Impedance Data for Mechanically Polished Bulk Samples of Titanium and Chromium(A) ␴C

C (F) Titanium Chromium (A)

–6

1.7 x 10 2.4 x 10–6

–8

1.8 x 10 2.4 x 10–8

␣

RS (Ω)

0.86 0.88

18 22

DISCUSSION

Electrode area = 0.2 cm2.

TABLE 3 Model Parameters from Impedance Data for Titanium, Chromium, and Titanium-Chromium Alloys(A)

Chromium Titanium Ti-20% Cr Ti-50% Cr (A)

C (F)

␴C

␣

RS (Ω)

2.9 x 10–5 3.8 x 10–5 2.8 x 10–6 2.5 x 10–6

4.4 x 10–6 5 x 10–6 3.4 x 10–5 2.8 x 10–5

0.82 0.89 0.94 0.95

21 20 18 18

␴C represents the standard deviation of the capacity determination.

(a)

(b) FIGURE 10. Bode diagram for chromium, titanium, and titaniumchromium alloy films deposited on silicon (electrolyte = 39 g/L NaCl, electrode rotation rate = 2,000 rpm, and temperature = 25°C).

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measurements decreased because statistical defects related to growth imperfections (e.g., dust particles) or to cracking dominated the obtained value of penetrating porosity.32 Therefore, more experiments will be necessary to elucidate the precise role of amorphous structure for penetrating porosity.

Results showed that the columnar grain structure of sputter-deposited chromium and titanium metal coatings caused significant surface roughness on a microscopic level. Alloying of chromium to titanium led to fewer columnar films and, in a certain concentration range, to an x-ray amorphous structure. Results agreed with those of Kim, et al.,1 who found that titanium-chromium alloys are amorphous in the concentration range from 37% Cr to 73% Cr. Amorphous structures for other alloy systems deposited by PVD have been observed by a number of authors, and it generally is believed that the amorphous structure is beneficial for the corrosion resistance of coatings.14-18 The micrograph of Figure 4(a) shows that the surface of the amorphous alloy was extremely flat, contrary to the pure metal coatings, which exhibited a growth-related surface texture (Figure 5). Therefore, when corrosion rates between coatings having different morphologies are compared, differences in surface roughness should be taken into account. Attempts to characterize the surface roughness of the metal coatings quantitatively with conventional stylus instruments or with the atomic force microscope (AFM) did not yield satisfactory results because the unfavorable amplitude/ wave length ratio of the roughness prevented the tip from following the surface profile accurately, resulting in extreme sensitivity of the measured roughness values to the shape of the tip.31 For this reason, EIS was used to measure real surface area. Application of EIS to determine real surface area of passive metal coatings has been discussed in detail previously, and it was demonstrated that it can yield meaningful values.29 The basic assumption of the method is that the interface capacity determined from impedance measurements is proportional to the real surface area of the electrode. The thickness of the charged layer at the interface responsible for the measured interface capacity must be smaller than the characteristic dimension of the surface topography. Under the experimental conditions used here, the corrosion potential always was situated in the passive potential region of the coating material. The measured value of C, therefore, was given by the capacity of the passive film and that of the double layer at the solid-liquid interface. The former was expected to dominate the measured value of C, but a detailed interpretation of the physical phenomena responsible for the measured capacity was not



needed to determine the surface area as long as the above assumption was fulfilled. The thickness of the electrical double layer in the concentrated electrolyte solutions used was typically on the order of 0.2 nm, while the thickness of passive films normally is on the order of 1 nm to 2 nm.36 Both values are much smaller than the typical dimension of the topographic features observed on crystalline PVD films (Figures 1[a] and 2[a]), which were on the order of a tenth of a micrometer. The value of the measured C, therefore, was proportional to the real surface area of the present coatings. When comparing the relative surface areas of alloys of different chemical composition using impedance, they should exhibit a similar electrochemical behavior in the applied potential range. According to Figure 8, the impedance diagrams for mechanically polished bulk samples of pure chromium and pure titanium almost coincided. Both metals showed an essentially capacitive behavior resulting from the presence of a stable passive film. Based upon results of Figure 8, it was reasonable to assume that the measured C of titanium-chromium was not affected significantly by composition. The data of Figure 10 showed that the impedance for PVD films exhibited a similar behavior as bulk metals. At high frequencies, the ohmic resistance dominated, and the amplitude of the impedance exhibited a plateau. Below a threshold frequency, which depended upon the capacity, the amplitude increased. The slope of the curve was constant and of similar value for all samples studied, which meant that only capacitive phenomena influenced the impedance in this intermediate range of frequencies. Data suggested that, even at low frequencies, the current through the interface essentially resulted from relaxation phenomena associated with a capacitive behavior of the passive film rather than from charge-transfer processes. These arguments led to the conclusion that, for the present system, capacity measurements could be used safely to compare relative values of real surface area of titanium, chromium, and titaniumchromium alloy films. Data of Figure 11 suggested that neither the process of alloying nor the amorphous structure had a significant effect on the intrinsic corrosion rate of PVD titanium-chromium alloy films when the relative surface area was taken into account. This observation was in apparent contradiction to findings of other authors14-17 and applied to the rate of uniform corrosion only. It did not exclude that an amorphous structure might be beneficial for resistance to pitting corrosion. Indeed, pits are initiated at weak points in the passive oxide film such as growth defects, grain boundaries, etc.37 The more uniform surface morphology of x-ray amorphous films and the absence of possible grain boundary segregation phenomena, therefore, should have a beneficial influence on pit


FIGURE 11. Measured corrosion currents for titanium, Ti-20% Cr, Ti50% Cr, and chromium films on silicon and computed corrosion currents taking into account different surface roughness (electrolyte = 39 g/L NaCl, electrode rotation rate = 2,000 rpm, and temperature = 25°C).

FIGURE 12. Polarization curves measured on sputtered titanium, chromium, and titanium-chromium alloy coatings sputter deposited on brass (electrolyte = 39 g/L NaCl, scan rate = 2 mV/s, electrode rotation rate = 2,000 rpm, and temperature = 25°C).

initiation even if the uniform corrosion rate is not affected. The effect of film structure on different corrosion mechanisms needs further study. When PVD coatings are to be used for the protection of a metallic substrate, porosity is the most critical factor. Linear sweep voltammetry was found to be well-suited for evaluation of the relative values of porosity of the present coatings deposited on brass. Results of Figure 12 showed that, for given deposition conditions, alloying titanium with 20% or 50% Cr significantly lowered porosity of the coatings compared to the pure metals. This observation is of practical interest for development of novel PVD duplex coatings involving an inner layer for corrosion protection.32 Among the two titanium-chromium alloys studied, Ti-20% Cr coatings were found to be slightly less porous than the amorphous Ti-50% Cr

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coatings. This suggested that amorphicity is not the sole factor to be taken into consideration for optimizing the porosity of PVD alloy coatings.

CONCLUSIONS ❖ Sputter-deposited titanium-chromium alloy films were found to differ in microstructure and surface topography from pure titanium and chromium films fabricated under the same conditions. In an intermediate concentration range, the alloy films were x-ray amorphous and exhibited an almost perfectly flat surface. ❖ Based upon the geometrical surface area, the corrosion current density determined from potentiodynamic experiments was found to be lower for the titanium-chromium alloy films than for pure titanium and chromium films. Impedance measurements explained this difference by a larger real surface area of the metal films. ❖ Electrochemical polarization measurements of coatings deposited on brass demonstrated that alloying titanium with chromium significantly lowered the penetrating porosity of sputter-deposited PVD films. ❖ Results showed that not only chemical effects but also the influence of co-depositing elements on film microstructure, which determines surface roughness and porosity, should be considered when studying the corrosion behavior of PVD alloy coatings.

ACKNOWLEDGMENTS Financial support for this work was provided by Fonds National Suisse. The authors acknowledge the assistance of P. Agarwal with interpretation of the impedance measurements. REFERENCES 1. J.H. Kim, E. Akiyama, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 34 (1993): p. 1,817. 2. E. Akiyama, H. Yoshioka, J.H. Kim, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 34 (1993): p. 27. 3. E. Akiyama, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Mater. Sci. Eng. A 181 (1994): p. 1,128. 4. G.S. Frankel, R.C. Newman, C.V. Jahnes, M.A. Russak, J. Electrochem. Soc. 140 (1993): p. 2,192.

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